An RF circuit component, including a resonator as its basic element, is an essential component of an RF communications system. For example, a mobile communications system needs an RF circuit component functioning as a narrow band filter to utilize its frequency range effectively. Also, in the base station of a mobile communications system and a communication satellite, the development of a narrow-band, low-loss and small-sized filter that can withstand huge power has been long awaited.
Also, milliwave or quasi milliwave band wireless communications systems, developed remarkably these days, have used waveguide filters but desperately need such small-sized and low-loss filters, too.
Some of currently used RF circuit components such as resonator filters adopt a transmission line structure. An RF circuit component with a transmission line structure is small-sized and applicable up to radio frequencies falling within the microwave and milliwave ranges. Also, such an RF circuit component has a two-dimensional structure defined on a substrate, can be easily combined with other circuits and components, and is used extensively today.
As a typical example of a planar transmission line structure, an RF circuit component, which makes a disklike resonator exhibit a filtering characteristic by coupling a dipole mode thereto with protrusions provided for portions of its outer periphery, was reported in U.S. Pat. No. 5,172,084, for example.
The present inventor invented a multi-stage filter such as that shown in FIG. 7 and disclosed it in Japanese Laid-Open Patent Application Publication No. 2000-77905. This filter includes three elliptical conductors 2a, 2b and 2c, which are arranged in line, and two coupling terminals 6a and 6b coupled to the elliptical conductor 2a.
This filter can create an attenuation pole in a curve representing a filter characteristic. However, it is still difficult even for this filter to create the attenuation pole at a desired frequency and with a desired quantity of attenuation. This is because the frequency and quantity of attenuation of the attenuation pole need to be adjusted according to the specific combination of the degree of coupling between the elliptical conductors 2a, 2b and 2c, the filter characteristic and the quantity of filter loss.
Japanese Laid-Open Patent Application Publications No. 8-46413 and No. 10-308611 disclose RF circuit components including a disklike or elliptical conductor as its resonator. However, it is difficult for each of these RF circuit components to control the transmission characteristic with high precision, which is a common problem for them.
In order to overcome the problems described above, an object of the present invention is to provide an RF circuit component that achieves a desired frequency and a desired quantity of attenuation with a simple configuration.